1. Field of the Invention
The present invention is directed to a magnetic field detector, particularly for implantable medical apparatuses, of the type having a coil mounted at the free end of a resilient member, which is fixed at an opposite end, and a detecting device for detecting, as a measure of the magnetic field, the movement of the coil, when fed with an electric current from a controllable current source, within the magnetic field which is to be measured.
2. Description of the Prior Art
U.S. Pat. No. 4,887,032 discloses a sensor for measurement of position, or sensing the passage of an object past a point, which sensor comprises a flexible bar member provided with a piezoelectric element. The bar has a free end of which a coil is supported, and is fixed at its opposite end. If the coil is located in a magnetic field, it will move if a current is passed through the coil and this movement of the coil will cause the piezoelectric bar to flex and a corresponding voltage will be produced by the associated piezoelectric element. This sensor is used for measuring the distance from an object to a reference point, or the rate at which an object passes a reference point to which object a magnet is attached or the object is formed of a material of magnetic permeability and is moving in a permanent magnetic field.
For implanted medical devices, like pacemakers, a magnet is often used for testing purposes. The battery status can, for instance, be detected by the application of a magnet to the implanted device. The exact location of the implanted device can often be difficult to determine and therefore it is also difficult to ascertain the correct location at which the magnet should be placed in relation to, e.g., a reed switch within an implanted pacemaker.
In European Application No. 91114252.9 (assigned to the same assignee, Siemens Aktiengesellschaft, as the present application, but as yet unpublished), a magnetic field detector for use in implantable medical devices is disclosed having a movement-sensitive sensor in the form of a piezoelectric plate, to which a coil is attached. The coil moves when fed with current and when located in a magnetic field, the movement of the coil causing the piezoelectric plate to be deformed, and thus generating a signal representative of the movement.